1. Field of the Invention
This invention relates to a cylinder for use in such applications as the driving of machines and conveyance of articles, and more particularly to a cylinder of the type that has no piston rod.
2. Description of the Prior Art
Various hydraulic cylinders dispensing with the piston rod through the use of magnets are known, such as the one disclosed in the U.S. Pat. No. 4,488,477.
A rodless cylinder of such a known type has series of driving and driven magnets axially slidably disposed inside and outside the cylinder tube thereof. The series of driving magnets is attached to a piston that is hydraulically moved inside the cylinder tube along the axis thereof. Said two series of driving and driven magnets are disposed so that unlike poles of the individual magnets are opposite to each other.
Having no rod attached to the piston, rodless cylinders of the type just described conserve much more space in the direction of stroke than common piston cylinders that come with rods. With a cylinder that comes with a piston rod, the length of the piston rod plus the cylinder becomes almost twice that of the stroke thereof when the piston rod projects from the cylinder to the maximum extent. As opposed to this, the overall length of a rodless cylinder is always substantially equal to that of the stroke thereof irrespective of the position of the piston.
When put to actual use, such as in a conveyor, however, provision must be made to prevent the series of driven magnets disposed outside the cylinder tube from revolving therearound. Here arises the need of providing guide rails or other similar rails along the cylinder tube so that the member to be guided thereby and the outer series of driven magnets are integrated. But this integration involves a disadvantage that the size of the driven member increases.
Also, piping to supply the pressurized fluid to drive the piston from both ends of the cylinder tube should be provided together with said rails. But such piping might offer an obstacle to the smooth execution of work.
In integrating the member to be guided by such rails and the series of driven magnets outside the cylinder tube, the driven member may consist of a block perforated with holes to pass the cylinder tube and rods, pipes or other appropriate members serving as the guiding rails. But this arrangement also involves some practical problems as pointed out below.
To begin with, the driven member cannot slide smoothly unless the cylinder tube and guiding rails are always precisely kept in a given positional relationship or in positions corresponding to the holes provided in the block. Keeping the cylinder tube and guiding rails in a given positional relationship not only is technically difficult, but also will prove extremely costly even when achieved. Then, to derive the maximum driving force from the cylinder, any load acting on the block making up the driven member should be supported not by the cylinder tube but by the guiding rails.